Mechanisms of Plasmodium departure from the liver infection stage

疟原虫需要肝脏有一个发育周期，然后才能感染血液并导致疟疾。由于被感染的蚊子叮咬传播的寄生虫数量有限，肝脏中只有很少的细胞会感染疟原虫。因此，成熟的肝期寄生虫需要有效地退出这些细胞，以建立血液中的感染。为了逃脱受感染的肝细胞并促进向血液的转移，肝脏阶段的裂殖子以被称为分体的结构排出。裂殖体是被质膜包围的裂殖子包，当出口被触发时，裂殖子从受感染的宿主细胞萌发。尽管已经描述了导致分体形成的步骤，但启动和驱动这些事件的分子活动仍然没有明确的定义。已知蛋白酶活性对肝期疟原虫的出口是必不可少的，但这种酶活性的分子后果尚不清楚。我们确定了富含丝氨酸的抗原(血清)家族的一个成员，PbSERA4，是有效退出肝期伯氏疟原虫所必需的。PbSERA4是一种可能的半胱氨酸蛋白酶，但其蛋白分解功能尚未得到实验证实。我们在预测的PbSERA4活性部位产生了具有特定点突变的寄生虫，这种变化，就像PbSERA4的缺失一样，延长了肝脏和血液生长阶段之间的过渡。这些结果证实了PbSERA4的蛋白分解活性是其在肝脏感染阶段达到顶峰所必需的。我们未来工作的重点之一是确定PbSERA4/PfSERA7蛋白酶的底物。我们计划采用多种基于蛋白质组学的互补方法来发现在疟原虫裂殖子排出过程中被切割的蛋白质。除了确定我们研究中确定的任何蛋白酶底物的作用外，我们还将检测在成熟的肝期寄生虫中表达的其他疟原虫蛋白，这些蛋白已知有助于通过肝脏进入血液。我们将调查这些候选基因所针对的寄生虫的表型，并评估蛋白分解处理在其功能中所起的作用。宿主细胞在疟原虫肝期裂殖子的退出过程中也起到了关键作用。我们发现神经酰胺类似物HPA-12限制伯氏疟原虫感染细胞的分体形成，这表明抑制宿主神经酰胺的运输和鞘磷脂的合成限制了肝期寄生虫的退出。我们将继续探索这一途径和其他宿主途径对肝期裂殖子出口的贡献，我们的合作方法将揭示疟原虫肝裂殖子的出口与使用非裂解出口策略的其他病原体的出口相比如何。这个多学科的项目将阐明控制疟原虫从肝脏排出的分子机制。

Plasmodium requires a developmental cycle in the liver before it can infect the blood and cause malaria. Because the number of parasites delivered by the bite of an infected mosquito are limited, only few cells in the liver become infected with Plasmodium. Consequently, mature liver-stage parasites need to exit these cells efficiently to establish the infection in the blood. To escape infected liver cells and promote the transition to the blood, liver-stage merozoites egress in structures known as merosomes. Merosomes are packages of merozoites surrounded by plasma membrane that bud from the infected host cell when egress is triggered. Although the steps leading up to merosome formation have been described, the molecular activities that initiate and drive these events remain poorly defined. Protease activity is known to be essential for liver-stage Plasmodium egress, but the molecular consequences of this protease activity are unknown. We determined that a member of the serine-rich antigen (SERA) family, PbSERA4, is required for efficient exit of liver-stage Plasmodium berghei. PbSERA4 is a putative cysteine protease; however, its proteolytic function had not yet been confirmed experimentally. We generated parasites with a specific point mutation in the predicted active site of PbSERA4 and this change, like deletion of PbSERA4, prolongs the transition between the liver and blood stages of growth. These results confirm that the proteolytic activity of PbSERA4 is required for its role at the culmination of the liver infection stage. One priority of our future work is to identify the substrates of the PbSERA4/PfSERA7 protease. We plan multiple complementary proteomics-based approaches to discover proteins that are cleaved during egress of Plasmodium merozoites. In addition to characterizing the role of any protease substrates identified in our studies, we will examine additional Plasmodium proteins that are expressed in mature liver-stage parasites and are known to facilitate the transition through the liver and into the blood. We will investigate the phenotype of parasites in which these candidates are genetically targeted and evaluate the role proteolytic processing plays in their function. The host cell also has a critical contribution in the exit of Plasmodium liver-stage merozoites. We found that the ceramide analog HPA-12 restricts merosome formation from P. berghei-infected cells, which suggests inhibition of host ceramide transport and sphingolipid synthesis restricts liver-stage parasite exit. We will continue to explore the contribution of this and other host pathways on liver-stage merozoite egress, and our collaborative approach will reveal how the exit of Plasmodium liver merozoites compares to the egress of other pathogens that use non-lytic egress strategies. This multi-disciplinary project will elucidate molecular mechanisms governing the exit of Plasmodium from the liver.